1. Field of the Invention
The invention relates generally to a spinal alignment system for treating spinal deformities and/or injuries and, more particularly, to securing a spinal alignment system to a spinal column.
2. Description of the Related Art
Deformities of the spine and injuries to the spine have long been treated by surgical intervention. In treating deformities and injuries to the spine, the goal is to return the spine, to the extent possible, back to a normal curvature and/or hold it in a desired position. Several surgical intervention techniques and devices have been proposed for the treatment of injuries to and deformities of the spine.
Some of these surgical techniques use a hook and rod system to hold the spine in a desired position. In such systems, the rod can be placed along the outside of the curved spine, that is on the convex side, and can be attached to the vertebrae of the spine by hooks as illustrated in FIG. 1. Because the rod is applying a generally compressive force along the spine to urge it into the desired position, this can be referred to as the compressive mode. Alternatively, a rod placed on the inside of the curved spine, that is on the concave side, would apply a generally tensile force along the spine to urge it into the desired position, which can be referred to as the distraction mode.
The device shown in FIG. 1 is an example of one of the first such systems referred to as a Harrington Instrumentation in the compression mode. With this device the rod is threaded (threads not shown) and can be straight or bent into a curvature that will place the spine in the desired position. Each hook is threaded onto the rod and secured to the rod by nuts and is frictionally seated on each vertebra. After all the hooks have been frictionally seated on the vertebrae and secured to the rod, the excess rod can be cut off. In the compression mode, the middle of the rod is pushing the spine to the desired position and the ends of the rod are pulling the spine to the desired position. This pulling force at the ends of the rod can be of such magnitude that the hook and rod system can fail. Depending upon the force, the hooks can pull or slip off because the hooks are open ended. Regardless, the desired position of the spine can be lost. Hook pull off and failure is not limited to the compression mode, it is also a problem in the distraction mode for this type of system. In addition, the ratcheted rods in the distraction mode could break at the ratchet points, again causing system failure.
To overcome problems with hooks pulling off the vertebrae, some devices attempted to secure the rod to the vertebrae with a screw screwed into the bone material instead of using a hook. The screw, however, is under perpendicular tension by the rod, thus, the pull on the screw is straight out of the bone along the screw's axis. With this construction, the screw is held in the bone only by the strength of its threads. Moreover, because the bone of the vertebrae is mostly cancellous and is relatively weak, it is not a preferred bone type for thread fixation. The weaker bone material is more likely to fail than the screw. As a result, the screws that are subject to the pulling forces described above can experience pull out and failure.
To avoid screw pull out due to bone failure, larger bone screws and larger threads have been used. But these larger features can disrupt too much of the vertebrae and cause the vertebrae to fracture, independent of screw pull out. Failures such as these can prevent further repair of the spinal column disorder unless there is additional surgery.
In a modification of the screw and rod system, the screws pass through plates that have slots or holes so that the screw can be oriented relative to the rod at an angle other than perpendicular to the rod. The slots or holes are configured to permit the angle of the screw relative to the rod to be selected from a range of angles. This modified orientation of the screw can reduce the load that the threads must support. However, the screw is still subject to failure by pull out and the threads can still cause the vertebra to fracture.
This effort to overcome problems experienced with screws led to a hook being used in combination with a screw. See, for example, U.S. Pat. No. 5,584,832. But use of both a hook and a screw does not eliminate the problems associated with each. The hook can still experience slippage and the screw can still experience pull out. In these devices, a hook frictionally engages the vertebra and a bone screw is passed into bone as well. Although these devices can provide for a more reliable mounting of the rod onto the spinal column over other devices, these devices, as stated above, cannot eliminate hook slippage and bone screw pull out. Also, this combination device requires a greater number of components for implantation as compared to the hook and rod systems and the bone screw systems discussed above.
In an effort to overcome problems experienced with screw pull out, a nut can be threaded on the far end of the screw. But nuts used on the far side of the screw in the spine are difficult to place and to hold until secure. The nut must be held blind on the side away from the incision. The screw needs to center in the nut, which requires palpation by the surgeon. The screw can be sharp, tearing gloves, exposing skin or even cutting the surgeon. Also, for the nut to relieve the screw threads from being the sole resistance force to pull out, the nut must be sufficiently tight against the bone, which is difficult to achieve by hand (without a holding device so that the nut does not spin while tightening is occurring). The use of a nut with the screw can be a difficult, cumbersome and time consuming procedure at best.
Furthermore, when using screws in spinal systems where the screw penetrates the vertebrae, with or without the use of a nut or hook, the screws must be accurately positioned on the vertebrae. This requires the use of some type of a jig or gauge or both. This holding device presets the target location for the screw to assure that the screws are precisely and accurately placed. A gauge is also used to measure the distance the screw should span, thus, avoiding over penetration beyond and into unintended tissues, and avoiding under penetration, which would mean fewer threads to resist pull out.
Accordingly, what is needed is an arrangement for securing a fixation rod of a spinal alignment system to the vertebra that is easy to install, yet is reliable and durable.